a11y: nav aria-label, search role=search + label, card hover motion guard

- templates/nav.php: add aria-label="Navigation principale" to <nav>; emit
  aria-current="page" on the active link alongside the existing CSS class
  so screen readers announce the current page without relying on colour/style alone

- templates/search-bar.php: add role="search" + aria-label="Recherche" to
  the <form>; add a visually-hidden <label for="site-search-input"> linked to
  the input via id="site-search-input", satisfying WCAG 3.3.2 (labels/instructions)
  and 4.1.2 (name/role/value) — placeholder text alone is not a label

- public/assets/main.css: add @media (prefers-reduced-motion: reduce) block that
  sets transition:none and transform:none on .card__media img/video hover, so the
  scale(1.02) zoom is fully suppressed for users who opt out of motion (WCAG 2.3.3 /
  prefers-reduced-motion); the global transition-duration guard in common.css already
  covers all other transitions but does not zero the transform value itself

Fixes TODO sections: G (nav/search-bar landmark names), I (site-search form ARIA),
3.3.2 (search input label), prefers-reduced-motion (card hover transform gate)

templates/nav.php

@@ -4,14 +4,17 @@
 // Provide $currentNav variable to mark active links (optional)
 $_navCurrent = $currentNav ?? '';
 ?>
-<nav class="site-nav">
+<nav class="site-nav" aria-label="Navigation principale">
     <a class="site-nav__logo" href="/index.php">Posterg</a>
     <div class="site-nav__links">
         <a class="site-nav__link <?= ($_navCurrent === 'repertoire') ? 'site-nav__link--active' : '' ?>"
-           href="/search.php">Répertoire</a>
+           href="/search.php"
+           <?= ($_navCurrent === 'repertoire') ? 'aria-current="page"' : '' ?>>Répertoire</a>
         <a class="site-nav__link <?= ($_navCurrent === 'licence') ? 'site-nav__link--active' : '' ?>"
-           href="/licence.php">Licence</a>
+           href="/licence.php"
+           <?= ($_navCurrent === 'licence') ? 'aria-current="page"' : '' ?>>Licence</a>
     </div>
     <a class="site-nav__link <?= ($_navCurrent === 'apropos') ? 'site-nav__link--active' : '' ?>"
-       href="/apropos.php">À Propos</a>
+       href="/apropos.php"
+       <?= ($_navCurrent === 'apropos') ? 'aria-current="page"' : '' ?>>À Propos</a>
 </nav>